The number of infectious illness outbreaks on cruise ships has dropped sharply in the last 12 months, greatly extending what had already been a stretch of smooth sailing for the industry.

Since July 2016, just four ships have experienced gastrointestinal illness that reached the level of an outbreak, according to the U.S. Centers for Disease Control (CDC), which monitors cruise ship outbreaks.

That compares with an average of 19 outbreaks annually from 2008 to 2014.

About 70% of the gastrointestinal outbreaks are caused by norovirus, a common malady that lasts for several days, is easily transmitted and comes aboard cruise ships with passengers who are already sick.

The virus causes outbreaks in many places where people congregate on land, such as schools and nursing homes, but only cruise lines have agreed to report the outbreaks to federal health authorities.

Last year, there were 13 episodes of illness on cruise ships, according to the CDC's website, but only two after mid-year. They occurred on the P&O Cruises ship Adonia, which was sailing for Carnival Corp.'s Fathom brand at the time, and on Holland America Line's Oosterdam.

This year, outbreaks have been reported only on Princess Cruises' Coral Princess, in March, and Oceania's Regatta, in April.

LaKia Bryant, a spokeswoman for the CDC, said the rate of gastrointestinal illness on cruise ships has decreased over time "as has the number and severity of outbreaks by year, except 2012, when a new strain of norovirus emerged."

Still, if the outbreaks continue at their current pace, 2017 will have by far the lowest number on record.

Bryant said that the trend is likely the result of several factors.

One is earlier detection of illness. Cruise lines have been refining their response to viral outbreaks for more than a decade and are becoming better practiced at identifying sick passengers and quarantining them before an outbreak can get started.

One line that has a particularly good track record is Carnival Cruise Line, which has only had one outbreak in the past six years.

Jennifer De La Cruz, vice president of communications at Carnival, said that over the past 10 years, "We have typically had either zero or one outbreak per year. We credit that to very close monitoring, tracking and rapid implementation of outbreak-prevention protocols if we start to see any unusual uptick in number of [gastrointestinal] cases."

Another factor, Bryant said, is cruise industry diligence in "developing and implementing ... outbreak prevention and control plans."

Cruise lines have adopted a number of measures to prevent outbreaks. At Holland America Line, for example, passengers are asked not to serve themselves at the buffet restaurant in the first few days of the cruise, but instead allow crew to handle the food.

Most newly built cruise ships for the past couple of years have included hand washing stations at the entrance to the buffet.

Bryant said such innovations were part of the industry "proactively looking for ways to use the most current science and limit the spread" of gastrointestinal illness.

The long-term decline in outbreaks at sea was documented in a January 2016 paper in the CDC publication Morbidity and Mortality Weekly Report.

Three health researchers affiliated with the CDC found that from 2008 to 2014, rates of acute gastroenteritis per 100,000 travel days decreased among cruise passengers, from 27.2 cases in 2008 to 22.3 in 2014.

The rates were lower than in a similar study of the 2001-to-2004 period.

Researchers found that of the 29,107 voyages during the period, 133 (0.5%) had reported an outbreak, and of those outbreaks where testing enabled verification, 92% were caused by norovirus.

The report went on to say that the seasonality of norovirus outbreaks is the same onboard ships as it is on land, with higher rates of illness reported during the November--through-April period each year.

This past winter, there were norovirus outbreaks reported by local health authorities in several California school districts, including one that infected at least 700 elementary and middle school students in Ventura County, north of Los Angeles.

The CDC said its collaboration with the cruise industry helps support efforts to contain outbreaks and spread knowledge about effective countermeasures. Individual cruise companies also share best practices.

Carnival Corp. chief communications officer Roger Frizzell said the company's brands share their own best practices, especially in the areas of health, environment, safety and security. He said that former Navy vice admiral Bill Burke, who joined Carnival several years ago as chief maritime officer, has focused on illness prevention, "and it is having an impact."

"The cruise industry as a whole has always done a great job in this area," Frizzell said. "In fact, the data shows the best way not to attract norovirus is to take a cruise."

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